The present invention is related generally to semiconductor fabrication and, more particularly, to a system and method for removal of photoresist in front-end fabrication as part of integrated circuit manufacturing.
A schematic of a standard configuration of a photoresist (PR) stripping chamber and source is shown in FIG. 1. Gas coming from a set of flow controllers and valves 101, passes via tubing 102 to a plasma source 103. There, the gas becomes substantially dissociated (and partially ionized) and then goes through a distribution/baffling system 104 into a wafer process enclosure 105. A pedestal 106 holds a wafer 107 which is to be stripped of PR and residues. On the wafer, radicals react with the PR and residues to form volatile or water-soluble reaction products that are then pumped out by ducts 108. This type of PR stripping chamber is widely used because it provides almost entirely neutral reactive species to strip the PR and does not subject the wafer to large amounts of charged particles that might damage the sensitive materials and layers used in making the integrated circuits. Such stripping systems are used for removing PR both during the early stages of IC fabrication, when the transistors are being fabricated, as well as the later stages when the interconnecting metal lines are made to connect the transistors in the desired circuit pattern and to external circuits.
There are several specific PR stripping applications done in the early stage of transistor fabrication, including stripping after ion implantation and stripping after etching used to pattern layers or make openings. The most critical of these stripping processes are those done after high dose ion implantation, after etching to pattern the gate electrode, and after etching to create openings to connect to the junctions and gate of the transistors. The latter are called the “contact” etches. PR removal processes for these applications may use a single step, but commonly employ two or even more steps. It is submitted that current PR stripping processes for these applications will become inadequate to meet all process requirements in the near future as the size of transistors continues to shrink and the thickness of critical layers on the wafer surface continues to decrease.
Processes for removing high dose implanted PR very frequently use multiple steps. The reason is the complex physical structure of the implanted PR. Usually, the first one or two steps are gas-phase done with apparatus such as in FIG. 1, while the final cleaning step for residues is often done with wet chemicals—usually in baths. The physical structure of the resist after high dose ion implantation is shown in FIG. 2, generally indicated by the reference number 200. A first layer 201 is a hardened “crust” which has received most of the ion dose when it was used as a mask during implantation. Research has shown that the crust is a graphitic, highly crosslinked, predominantly carbon polymer when there is sufficient dose of ions. The remaining PR (so called “bulk”), which has not been ion implanted, is indicated by the reference number 202. It can be seen that the crust and the bulk PR contact silicon 203, with the crust partially or completely enveloping the bulk. When wafers are stripped following high dose implant (HDI) in a single high-temperature (>200 Celsius) step, “popping” of the photoresist crust occurs. In this “popping”, the high temperature causes solvents contained in the bulk resist to go into the gas phase and pressure to build up underneath the crust. When the pressure builds sufficiently, it causes the hardened crust to rupture or detach from the silicon to vent the built-up pressure. In single step stripping of such HDI PR, popping is tolerated and ignored. Once the crust pops, both the bulk PR and the crust etch much more quickly in standard oxygen-based stripping processes—normally done between 200 Celsius and 300 Celsius. The particles that remain after such a single-step dry stripping process are usually physically and chemically removed by a wet bath clean, often including physical agitation such as Megasonic.
However, since the structure of the PR after implantation is two-layered with very different material characteristics, it is often the case that two or more gas-phase process steps are used to remove it. This is especially true in cases where it is important to reduce particulate levels following the stripping. One reason is that hardened crust, unlike bulk PR, is very hard to strip with normal reactive radicals. In most multistep stripping processes, the first step removes the crust at reduced wafer temperature (<120 Celsius) to avoid popping. Because of the low wafer temperature, additional reaction activation mechanisms need to be provided to achieve adequate stripping rate and productivity. One such method is to use gaseous additives, which accelerates the etching of the carbon polymer crust. Otherwise the stripping of crust in pure oxygen is often so slow that the cost is prohibitively high. Such additives may include fluorinated gases or mixtures containing hydrogen and nitrogen. Such gas mixtures passed through the plasma source produce other reactive species in addition to atomic oxygen which promote the attack of the crust material—as found by Hirose in Journal of the Electrochemical Society, 1994. Such methods, however, still do not yield fast and highly productive etching of the crust. Therefore, other techniques for activating the etching of the crosslinked carbon polymer crust are often needed. In many such processes, ion-bombardment has been used to provide such activation energy to accelerate the etching of the crust.
The second step to remove the remaining bulk PR (which has not been chemically altered during the ion implantation) is done at higher wafer temperature than the crust etching step—normally at least 200 Celsius—to speed up the etching rate. Thus, after the crust is breached and there is no further danger of popping, since pressure cannot build up, the temperature may be higher. At this stage, the bulk PR is accessible to the gas phase species and such photoresist, not altered by ion implantation, quickly etches chemically. This step is very similar to normal PR stripping applications, since the underlayer of photoresist has been substantially unchanged by the patterning or doping process.
However, the stripping of the bulk photoresist is not normally the final step, due to the significant side-effects of ion implantation. Very often, there are difficult to remove residues remaining after removing most of the crust and all the bulk PR. Such residues are usually made up of the remnants of the implanted resist crust (carbon polymer, dopants and silicon oxide). Usually, silicon compounds are found mostly on the sidewalls of the photoresist, whereas most dopants are found deep inside the crust. The ion implantation process always causes some silicon dioxide or silicon exposed on the wafer surface to be sputtered. Such sputtered silicon will, in part, strike and condense on the sidewalls of the photoresist structures leaving one or more monolayers of silicon or silicon dioxide on the outer surface of the sidewall crust. This material does not normally (especially in oxygen-based stripping) chemically convert to loose and soluble form during the bulk resist step and may be chemically quite resistant. Therefore, a necessary final step in removal of implanted PR is often the residue removal step. This may or may not be done in the same system as the first two steps because it may in part require use of wet chemicals including strong acids or bases. When it is done in part in the same system used for crust and bulk stripping, it usually involves the use of gas additives to the normal oxygen gas injected into the plasma source. Common additives are fluorinated gases as well as mixtures containing nitrogen and/or hydrogen, particularly “forming gas” (FG) which is at least 90% nitrogen or noble inert gas and the remainder hydrogen. Such additives help to convert the residues to a water-soluble form that will be removed by a deionized (DI) water rinse. It is common for such residue removal steps to still leave some un-reacted residues that are not removed in a DI water rinse. Such remnant residues will need to be removed by a wet chemical bath since the wafers need to be completely clean prior to the following process, high temperature thermal annealing of the dopant in the silicon.
Stripping processes following the “gate” etching (polysilicon for the current and next IC technology generation) step have typically been two step processes as well since there are almost always substantial silicon-containing residues remaining after the PR is stripped using the normally oxygen-based gas mixture. Such residues have sometimes been removed by adding to the injected mixture of gases a small flow of fluorinated carbon gas such as carbon tetrafluoride or hexafluoroethane. These provide small concentrations of atomic fluorine and other reactive species that chemically attack the silicon-containing residues and either remove it or leave a loose, water-soluble ash on the surface. Unfortunately, fluorine-containing gas addition has been shown in high temperature stripping processes to attack the gate dielectric—typically silicon dioxide or oxynitride. Fluorine atoms thus are passing right through the polysilicon gate electrode material to react with the dielectric. Sometimes a gas mixture using forming gas has been substituted for fluorinated gas so that there is a reduced effect on the gate dielectric material. The potential vulnerability of the gate and gate dielectric materials to the stripping process will likely increase as technology advances, due to the decreasing length of the gate which is typically about half the size of other critical features patterned on the wafer.
Last among critical stripping processes are those to remove PR following the contact etch. Contact etching processes are designed to stop when they reach the silicide or metal materials in the junctions and gate. Such materials will be changing over the coming generations of semiconductor technology. The dielectric that has been removed to make the holes is silicon dioxide. This must be etched to completion—often involving etching different amounts for holes connecting with gate versus source and drain—while not etching the silicides that are exposed at the bottom of the holes which have finished etching. Polymer residues containing silicon are very often left, following this etching, which need to be removed by the PR stripping process. Ideally, stripping processes will need to remove the PR and the residues without causing any loss of, harm or degradation of the materials exposed at the bottoms of the holes. Unfortunately, most silicide materials used for junctions including cobalt silicide and nickel silicide are sensitive to oxygen and degraded in performance by it. Two step processes may be used to successively remove the remaining PR after etching and then the residues that have substantial inorganic content.
Gas mixtures containing mainly oxygen have been the principal types of recipe used for all major stripping applications in transistor fabrication as part of IC manufacturing. This has been true for both the early patterning and ion implant steps involved in transistor fabrication as well as the later steps involved in making the interconnects or wires of the IC. Oxygen has been the gas of choice because atomic oxygen reacts more strongly with organic polymers like PR than most other radicals and is made from a very inexpensive gas that makes the process less expensive than when using other gases. Water vapor also produces high stripping rates but is not as inexpensive to deliver in gaseous form at high flow rates as is oxygen. Higher reactivity of species makes stripping rates faster, and faster rates make stripping system productivity higher. Such high rates have been an economic necessity for competitive stripping for many years because photoresist thickness for older lithography technologies (preceding Deep Ultraviolet lithography at 248 nanometers) has been greater than a micron or more. Since there are typically twenty or more photoresist removal steps in the IC manufacturing process, high stripping rates and productivity is needed in stripping to keep IC costs low for mass-market products. This means that stripping rates have needed to be several microns per minute or more.
Furthermore, until recently, surfaces exposed on the wafer during transistor fabrication (typically silicon dioxide) have been relatively insensitive to small amounts of silicon oxidation or silicon oxide loss. This made oxygen the preferred, safe and economic major gas ingredient for downstream plasma-based stripping of photoresist. Common gaseous additives to oxygen in such processes to improve process productivity and facilitate removal of residues during or following PR removal have included hydrogen or dilute mixtures of hydrogen in nitrogen (called forming gas) and fluorinated gases or mixtures of both. It has been found that small amounts of nitrogen/hydrogen gas additives modestly improve PR stripping rate.
Typically fluorine containing gas or dilute hydrogen mixtures such as forming gas is used following resist removal to help convert residues to forms soluble in water. Because such additives have not caused sufficient loss of silicon dioxide or damage to the silicon underneath it to adversely affect IC yield or reliability, addition of small percentages of such gases as FG or hydrogen or nitrogen or fluorine containing gases has been acceptable.
Gas mixtures having little or no oxygen or oxygen containing gas have been used with plasma-based systems since the early days of PR stripping where materials vulnerable to oxidation have been exposed on the wafer. One alternative to oxygen-based feed gases for stripping is hydrogen. In the early days of semiconductor IC fabs, hydrogen was employed as the main gas for stripping photoresist for some selected steps, during electrical interconnect formation, in the overall integrated circuit fabrication process to avoid oxidation of exposed interconnect metal on the wafers. Such an interconnect metal may include, for example, aluminum. This is currently the case for interconnect fabrication on integrated circuits where conducting wires on the wafer are made from copper. It is also true for other new materials such as low-k dielectrics. Consequently, processes employing high hydrogen concentration with no added oxygen are commonly used in the later stages of integrated circuit manufacture where copper and low-k dielectrics are exposed to the stripping reactive species (see for example, U.S. Pat. No. 6,630,406 issued to Walfried, et al.). In these processes, the hydrogen may also be used for reducing copper surfaces oxidized in previous steps.
Gas mixtures using hydrogen-containing gases with no oxygen have also been used for wafer surface treatments to avoid corrosion. In most cases, this was because metal surfaces or metal-containing residues left after stripping would form undesirable compounds on the surface of the wafer that would degrade the yield or performance of the IC. This and most other applications employing gas mixtures lacking oxygen have been steps in the fabrication of interconnects or wires between transistors.
High dose implanted PR crust etching using hydrogen rather than oxygen was first done prior to the sub-micron semiconductor technology era in a few factories. Conventional reactive ion etching systems (RIE) using some hydrogen-containing gas mixtures (principally a mixture of mostly nitrogen and a small percentage of hydrogen called “Forming Gas”) were found to be capable of removing the hardened crust on the PR surface formed by high dose ion implantation, at low wafer temperatures to avoid “popping”. The hydrogen-based treatment reduced the number of particulate defects found on the wafer surface after stripping the implanted PR. In particular, when the RIE systems used a small percentage of hydrogen gas highly diluted in nitrogen it successfully removed the crust so that a following step, usually employing a flow of oxygen radicals could then remove the remaining bulk PR. We have recognized that this worked because the wafer bombardment by energetic ions from the plasma provided energy to activate chemical reactions of radicals with the carbon polymer. This crust removal was done at low temperature to avoid popping of the crust, while the second step of bulk PR removal was preferably done at higher temperature, preferably with oxygen, to provide higher productivity. Such a gas mixture, having a small percentage of hydrogen in nitrogen [i.e., forming gas], was a safe and commonly used gas in semiconductor factories and considered safe for use in conventional PR stripping systems. Further, layer thicknesses such as, for example, silicon dioxide screen layers were extremely thick in the then-existing technology, on the order of 100 Angstroms, such that a considerable amount of damage or loss due to sputtering could be tolerated. However, concerns for problems such as wafer charging or contamination prevented the hydrogen-based processes using RIE from being commercially successful even in the earlier generations of IC fabrication technology.
The present invention resolves the foregoing difficulties and concerns while providing still further advantages, as will be described.